The prior art has recognized that improvements in efficiency could be obtained from properly directing the flow of water into a propeller. Previous systems had vane (stator)-propeller combinations in which the vanes were located either forward of the propeller (pre-swirl), or aft of the propeller (post-swirl). These systems have one or more of the following in common:
1. the vanes are mounted axisymmetrically, and are designed for the case in which the flow is perpendicular to the propeller disc. PA0 2. The vanes are designed to work in the viscous boundary layer of the ship; in this respect the vanes are operating as a flow directing device only. PA0 3. A specially designed propeller (not a modified commercially available off-the-shelf propeller) is used with either of the above vanes.
Recently, however, it has been recognized that in some cases the flow directing means should not be symmetrical since the flow into the propeller is not symmetrical. See, for example, Japanese patent application number 56-162006 (found in U.S. class 440 subclass 66) which shows a ship having a single propeller and which has a set of flow-directing vanes on only one side of its stern, the purpose of which is to create a wake stream flowing in the opposite direction to the turning direction of the propeller. The propeller shaft of the ship is horizontal; the flow distortion that the vanes are intended to overcome is caused by the boundary layer close to the hull.
Japanese patent application number 58-77998 (also found in U.S. class 440 subclass 66) shows a ship having dual propellers mounted on struts, one on each side of the stern. In this application the struts, which are asymmetrically arranged around the propeller disc, are contoured to provide water flow to the propeller with a rotary component opposite to the rotation of the screw propeller. However, in this application the propeller shafts are also horizontal with the flow distortion being caused by the shape of the stern of the vessel.
What the prior art has failed to recognize is that the flow into a propeller that is mounted on an inclined shaft is oblique and causes a once-per-revolution variation in propeller blade section angle of attack. The prior art also failed to recognize that the propeller on an outboard motor is also inclined to the water flow when the boat is moving.